1. Field of the Invention
The invention relates to the polarization of light within the deep UV spectrum at wavelengths of less than 250 nm, particularly at high power densities above 5 milli-joules per square centimeter per pulse.
2. Description of the Related Art
Polarizing ultraviolet light at wavelengths below 250 nm, particularly at power densities above five milli-joules per centimeter squared per pulse, presents special challenges. The typical optical materials for longer wavelength applications are either insufficiently transmissive or subject to breakdown at the short wavelengths and high power densities. Excimer lasers operating at 157 nm can be arranged to cut through all known solids including diamonds. Thus, the challenges for conveying or manipulating such light at somewhat lower concentrations are high.
Traditional polarizers are available in a number of different designs. These include cemented birefringent crystals in different relative orientations. The birefringent crystals have different effective refractive indices for different directions of polarization. The result is that one polarization component is refracted in one direction and the orthogonal polarization component is refracted in another direction. To function properly, the two crystals must be precisely oriented with respect to each other. Generally, this is done by cementing the two crystals together at a common interface. However, such polarizers cannot be used in the specified UV energy regime, because the UV radiation breaks down all of the known cements.
A similar polarizer design omits the cement by forming the mating surfaces between crystals with a high degree of flatness so that local polar forces between intimately related surfaces hold the crystals together. However, even such precisely mating surfaces have defects containing atmospheric materials that suffer slight surface absorption, which causes heating and subsequent delamination.
Another type of polarizer orients a succession of plates at Brewster's angle to the incident light. At each leading interface, one of the polarization components, i.e., the P polarization component oriented within the plane of incidence, is transmitted while a significant portion of the orthogonally related polarization component, i.e., the S polarization component oriented normal to the plane of incidence, is reflected. The plates are stacked one after the other so that each subsequent plate renders the transmitted beam more P polarized. However, some distortion occurs at each of the reflective interfaces, which can spatially disperse the incident light. The spatial dispersion alters the angle of incidence at which the light approaches subsequent plates, degrading performance and causing losses. In addition, the design is bulky, sensitive to collimation errors and can be optimized for only a single wavelength.
A polarizer known as the “MacNeille polarizer”, features a coating at a prism-to-prism interface formed by two coating layers having refractive indices that differ from each other and from the refractive indices of the adjacent prisms. The typical performance of the MacNeille polarizer includes a certain wavelength bandwidth over which reflection of the P polarization component remains near zero, while the reflection of the S polarization component is substantial. A substantial difference in the refractive indices of the two coated layers is required to achieve this type of performance. However, comparable coatings are not available for use a within the specified high-power, short-wavelength regime contemplated for the invention. The indices of the available coatings are too close to each other.
Another known type of polarizer is based on a thin film (quarter wavelength) dielectric stack. Here, alternating quarter-wavelength layers of differing refractive indices are deposited on a substrate. Although the reflectivity characteristics of both polarizations tend to parallel one another, there are gaps at limited wavelength ranges where one polarization is reflected much more than the other. However, the coating materials typically used for making such quarter-wavelength layers are not available for use in the high-power, short-wavelength regime contemplated for the invention. Again, the materials that are available tend not to differ very much in refractive index.